Adhesion-promoting surface

ABSTRACT

An article is provided, the article including a substrate having a surface, a nano-structure array formed on the substrate, the nano-structure array including a plurality of nano-structures extending from the surface of the substrate, and a cover layer formed on and around the nano-structures to anchor the cover layer to the substrate.

BACKGROUND

The present disclosure relates generally to adhesion to surfaces, andmore particularly, to the formation of nano-structures on a surface topromote adhesion.

Adhesive bonding is an alternative to the more traditional mechanicalfastening methods of joining materials, such as nails, rivets, andscrews. One of the major differences between an adhesive joint andmechanical fastening is that, generally, in mechanical fastening one orboth of the parts or materials being held together is pierced by amechanical fastener, whereas an adhesive joint may be formed without thepiercing the materials. This leads to one of the advantages of adhesivesover mechanical fastening, namely the ability to, not only fastendifferent materials, but to also to form a seal between components in asingle step. Mechanical fastening typically requires separate sealingand fastening steps to create a sealed part.

For example, in the area of microfluidics, the utilization of separatemechanical fasteners and sealants or gaskets would result in larger,more expensive, and less efficient devices compared to that obtainableusing an adhesive. Adhesives also provide an advantage in fasteningdissimilar materials together, from the standpoint of fasteningmaterials such as glasses, ceramics, and silicon devices, in whichforming the holes to allow fasteners to be utilized is difficult andexpensive.

In an inkjet printing system, a printhead structure may include a numberof discrete components connected via adhesive joints to define aprinting fluid path. The adhesive joints may be exposed to potentiallycorrosive printing fluids which, over time, may tend to weaken theadhesive joints, particularly at the interface between the adhesive andthe surface. Where an adhesive joint fails, printing fluids maypenetrate into regions where there is active circuitry, leading tocorrosion or electrical shorting, or both.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the present disclosure willbecome apparent by reference to the following detailed description anddrawings, in which like reference numerals correspond to similar, thoughperhaps not identical, components. For the sake of brevity, referencenumerals or features having a previously described function may or maynot be described in connection with other drawings in which they appear.

FIG. 1 is a perspective view of a schematic depiction of an articleformed in accordance with an embodiment of the present invention.

FIG. 2 is a cross-sectional view of the article shown in FIG. 1, takengenerally along line 2-2 of FIG. 1.

FIG. 3 is a cross-sectional view of an article including an array ofcapped nano-structures formed in accordance with an embodiment of thepresent invention.

FIGS. 4A through 4G schematically depict a method for fabricating anarticle having adhesion-promoting nano-structures formed in accordancewith an embodiment of the present invention.

FIG. 5 is a flowchart showing a method of adhering a cover layer to anarticle in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Referring initially to FIG. 1, an article 10 is shown, the depictedarticle including a substrate 20. In accordance with our teachings,substrate 20 defines a surface 22 with an array of nano-structures 30formed thereon, the nano-structures being configured to enhance surfaceadhesion of the substrate.

As shown in FIGS. 1 and 2, surface 22 may be a multi-faceted surface,and thus may define a first surface region 22 a and a second surfaceregion 22 b. The first and second surface regions intersect at an anglethat is less than 180 degrees. In the present example, the first andsecond surface regions intersect at an angle of approximately 90degrees.

Nano-structures 30 extend substantially orthogonally from the substratesurface 22. More particularly, a first set of nano-structures 30 aextend substantially orthogonally from first surface region 22 a in afirst direction (A) and a second set of nano-structures 30 b extendsubstantially orthogonally from second surface region 22 a in a seconddirection (B). Nano-structures may be formed on substantially all ofrespective surface regions, or may be formed on only a portion (orportions) of the surface regions, depending on the particular adhesionrequirements. In the present example, because the angle between firstsurface region 22 a and second surface region 22 b is less than 180degrees, it will be understood that first direction (A) intersectssecond direction (B).

As indicated, a cover layer 35 may be applied to at least a portion ofsubstrate surface 22, the cover layer typically being applied as aflowable material that substantially envelops nano-structures 30. Insome embodiments, cover layer 35 may take the form of an adhesive suchas SU-8, which is an epoxy-based negative photoresist manufactured byMicroChem Corporation. SU-8 is commonly used in the fabrication ofmicrofluidic devices such as printer printheads. Once applied, the SU-8may be solidified (e.g., by curing), securing the cover layer to thesubstrate surface 22 though chemical and/or mechanical means. Althoughan SU-8 cover layer is described in the present examples, cover layersformed of other materials may similarly be employed.

A variety of factors may contribute to securement of cover layer 35 tothe substrate 20. For example, because substrate surface 22 includesnano-structures 30, the surface area of the substrate surface isincreased relative to the surface area of a smooth substrate surface(e.g., an otherwise identical substrate surface without nano-structures30 formed thereon). The taller the nano-structures, the greater thesurface area of substrate surface 22 that will be exposed to contactwith cover layer 35. This increased surface area may provide a greaterarea for chemical bonding between the cover layer 35 and the substratesurface 22. Also, the chemistry of the nano-structures may be changed toaccommodate chemical bonding by, for example, applying a thin layer of asuitable adhesion-promoting material to the nano-structures bytechniques such as atomic layer deposition, adsorption,impregnation-sintering, etc.

Where the substrate includes intersecting surface regions (as shown inFIGS. 1 and 2), the nano-structures also may act to mechanically lockthe cover layer to the substrate. Referring to FIG. 2, for example, itwill be noted that the nano-structures 30 a may restrict movement of thecover layer in the second direction (B), and that nano-structures 30 brestrict movement of flowable material 35 in the first direction (A).Such restrictions are due, at least in part, to the reliable formationof nano-structures that extend substantially orthogonal to the substratesurfaces along intersecting trajectories. Such nano-structures opposeinterface shift (also referred to as “shear”) between the cover layerand the substrate surface in intersecting planes, thus locking coverlayer 35 in place.

FIG. 3 shows yet another feature that may contribute to securement ofcover layer 35 to the substrate 20. As indicated, substrate 20 may beprovided with nano-structures 40 having geometries that promotemechanical anchoring of the cover layer 35 to the substrate. Forexample, nano-structures 40 may take the form of capped nano-pillars,having stem portions 42 and cap portions 44. Stem portions 42 may becharacterized as having a stem diameter (d1), and cap portions may becharacterized as having a cap diameter (d2), where the stem diameter isnarrower than the cap diameter. Cap portions thus will tend to resistmovement of the cover layer in a direction away from substrate 20.

Methods disclosed herein may be used to control various properties ofthe nano-structures so as to promote adhesion through chemical bondingand/or mechanical anchoring. For example, nano-structures may bereliably formed orthogonal to the substrate surface, regardless of themorphology, geometry and/or orientation of the substrate.Nano-structures also may be reliably formed with geometries and/ordimensions (e.g., height, shape, etc.) that promote adhesion of thecover layer to the substrate. Even placement of the nano-structures maybe controlled using the methods disclosed herein.

The geometry of the nano-structures may be controlled so that thenano-structures have substantially uniform shape. Similarly, as shown inFIG. 2, the nano-structures may be substantially uniform in height (H),and the pitch of the nano-structures (the center-to-center distancebetween nano-structures (D)) may be substantially uniform.Nano-structures 40 thus may be substantially uniformly spaced across atleast a portion of the substrate surface, providing a substantiallyuniform nano-structured surface. Dimensions of nano-structures generallywill vary by less than 10% to 20% (for nanometer scale dimensions), andin some examples, may vary by as little as 1% or 2%.

Referring to FIG. 3, it will be appreciated that nano-structures 40 eachinclude an elongate stem portion 42 extending from the substrate and anelongate cap portion 44 extending from the stem portion. The examplestem portions take the form of cylindrical columns, each generallycharacterized as having a stem diameter (d1) and a stem height (h1).Stem portions 42 may have substantially uniform stem heights (h1), andmay have substantially uniform stem thicknesses (represented by stemdiameter (d1)) along such stem heights. The example cap portions 44similarly take the form of cylindrical columns, each generallycharacterized as having a cap diameter (d2) and a cap height (h2). Capportions 44 also may have substantially uniform cap heights (h2), andmay have substantially uniform thicknesses (represented by cap diameter(d2)) along such cap heights.

In the depicted example, the stem diameters (d1) are less than the capdiameters (d2), giving the nano-structures a generally “T” shape. SuchT-shaped nano-structures may enhance adhesive properties of thesubstrate, the broader caps serving to mechanically anchor cover layer35 to substrate 20. Although the example nano-structures have stemheights (h1) that are greater than cap heights (h2), cap height (h2) maybe greater than stem height (h1). The nano-structures similarly may haveother geometries, which may be determined at least in part by parametersof the fabrication process described below.

FIGS. 4A-4G depict an article 10 through various stages of fabrication.As shown, a substrate 20 thus may be adapted, through the presentmethod, to include a nano-structured surface that includes an array ofcapped nano-structures. Although a particular nano-structure geometry isshown, it will be understood that the fabrication process parameters maybe altered to achieve different nano-structure geometries.

Referring initially to FIG. 4A, fabrication begins with a substrate 20having a surface 22. Substrate 20 may be selected based, at least inpart, on the application for which article 10 will ultimately be used.If, for example, article 10 is to be used in a printer printhead (wheresemiconductor devices are to be formed on the substrate), the substratemay be formed from suitable support structures for semiconductors, suchas a substantially planar silicon wafer.

Substrate 20 similarly may be formed from other materials, e.g., glass,quartz, alumina, stainless steel, plastic, and/or the like, and may takeany of a variety of forms, including a multilayer structure and/or astructure with a non-planar surface (as shown in FIGS. 1 and 2). In thepresent example, a substantially planar substrate is shown (meaning thatthe surface is flat but may contain some irregularities).

As shown, a first oxidizable material is deposited on substrate 20 toform a layer of first oxidizable material 50. The first oxidizablematerial layer 50 may be formed using any suitable deposition techniqueknown in the art. Some non-limiting examples of suitable depositiontechniques include physical vapor deposition (PVD) (such as sputtering,thermal evaporation and pulsed laser deposition), atomic layerdeposition (ALD), or, in some instances, chemical vapor deposition(CVD).

In some examples, the first oxidizable material layer 50 may be formedof a metal or metal alloy that forms a dense metal oxide afterelectrochemical oxidation. Suitable oxidizable materials includeoxidizable refractory metals such as tantalum (Ta), niobium (Nb),titanium (Ti), tungsten (W), or their alloys. Such oxidizable materialsall can be electrochemically and/or thermally oxidized, and all haveexpansion coefficients (the ratio between thickness of the grown oxideand thickness of the consumed material) that are greater than 1.

In the present example, first oxidizable material layer 50 is formed oftantalum (Ta), which has been found suitable for use in the methodsdescribed herein. The example first oxidizable material layer also isreferred to herein as the “Ta layer”. The Ta layer may have any suitablethickness that will produce (during electrochemical oxidation) enoughoxide to form the nano-structures (which will be described in furtherdetail below). In some examples, the thickness of the Ta layer may beapproximately 100 to 1000 nanometers.

Referring still to FIG. 4A, it will be noted that a second oxidizablematerial is deposited on the Ta layer to form a layer of secondoxidizable material 60. The second oxidizable material layer may have athickness selected to produce a porous oxide (as described below), whichcorresponds to the desired nano-structures to be formed. The secondoxidizable material may be aluminum (Al), or may be an aluminum alloysuch as an alloy having aluminum as the main component. Secondoxidizable material layer 60 also is referred to herein as the “Allayer”. The Al layer may have any suitable thickness that will produce(during electrochemical oxidation) enough oxide to form a templatesufficient to produce the intended nano-structures. In some examples,the thickness of the Al layer may be approximately 10 to 1000nanometers.

Deposition of the second oxidizable material layer on the firstoxidizable material layer may be accomplished using any suitabledeposition technique known in the art. Some non-limiting examples ofsuitable deposition techniques include physical vapor deposition (PVD)(such as sputtering, thermal evaporation and pulsed laser deposition.

As shown generally in FIG. 4B, the multi-layer structure of FIG. 4A maybe further processed to form a nano-structure template 80 on substrate20. The nano-structure template defines a plurality of nano-pores 82,each having a first width (indicated as nano-pore diameter (d_(p1)), inthe present example). Such nano-pores are suitable for use in formingnano-structures on the substrate, as will be described herein.

In some examples, further processing includes a first anodizationprocess whereby second oxidizable material layer 60 (FIG. 4A) isanodized to define a plurality of substantially uniform, cylindricalnano-pores 82. Such nano-pores may be formed by completely anodizing thesecond oxidizable material layer 60 (e.g., the Al layer) so as toproduce a nano-structure template 80 in the form of a layer of porousoxide (e.g., anodic porous alumina, Al₂O₃) with nano-pores 82. Completeanodization refers to the oxidation sufficiently through the thicknessof the layer being anodized to allow anodization of underlying firstoxidizable material layer 50, as will be described below.

Anodization (i.e., electrochemical oxidation) is a process of forming anoxide layer on a material by making the material the anode in anelectrolytic cell and passing an electric current through the cell.Nano-pores are formed by field-assisted dissolving of the anode material(e.g., aluminum). Because field-assistant dissolving of anodic aluminastarts from the alumina-aluminum interface, the resulting pores arereliably orthogonal to the substrate surface, regardless of themorphology, geometry and/or orientation of the substrate surface. Foranodization of aluminum, as in the present example, applied voltage maybe kept constant at voltage within a range of about 10V to 200V. In someexamples, the first anodization process may occur at a voltage of about30V.

Geometry of the nano-structure template 80 may be adjusted by varyingone or more of anodization voltage, current density and electrolyte.Such adjustments to the first anodization process may alter nano-porepitch (D_(p)) and/or nano-pore diameter (d_(p1)), which characteristicsare illustrated in FIG. 4B. For example, nano-pore pitch may be relatedto anodization voltage, where nanometer pitch (D_(p)) is 2.8 nanometresper volt of anodization voltage. Nano-pore pitch (D_(p)) generally maybe adjusted within a range of from about 30 nanometers to about 500nanometers. Nano-pore diameter (d_(p1)) generally may be adjusted withina range of from about 10 nanometers to about 350 nanometers.

Anodization can be performed at constant current (galvanostatic regime),at constant voltage (potentiostatic regime) or at some combination ofthese regimes. Nano-pore diameter (d_(p1)) is proportional toanodization voltage. Accordingly, a potentiostatic regime may beemployed to produce a porous substrate with nano-pores havingsubstantially uniform nano-pore diameter (d_(p1)). Substantially uniformnano-pores 82, in turn, will yield substantially uniform nano-structures40, as will be described below.

The first anodization process may be carried out by exposing Al layer 60to an electrolytic bath containing an oxidizing acid such as sulfuricacid (H₂SO₄), phosphoric acid (H₃PO₄) oxalic acid (C₂H₂O₄) and/orchromic acid (H₂CrO₄). The electrolyte may be present, for example, in awater-based solution. The voltage applied during the first anodizationprocess may be selected based on the electrolyte composition. Forexample, the voltage may range from 5-25V for an electrolyte based onsulfuric acid, 10-80V for an electrolyte based on oxalic acid, and50-150V for an electrolyte based on phosphoric acid. The particularvoltage used will depend on the desired pore diameter (and thesuitability of such voltage for the electrolyte).

Aano-pore diameter (d_(p1)) also is related to the nature of theelectrolyte used. Accordingly, an electrolyte may be selected to achievea particular desired nano-pore diameter (d_(p1)). As non-limitingexamples, nano-pores 82 of the following sizes may be obtained using thefollowing electrolytes: nano-pore diameters (d_(p1)) of about 20nanometers may be obtained using H₂SO₄ (in a water-based solution) asthe electrolyte; nano-pores diameters (d_(p1)) of about 40 nanometersmay be obtained using C₂H₂O₄ (in a water-based solution) as theelectrolyte; and nano-pores diameters (d_(p1)) of about 120 nanometersmay be obtained using H₃PO₄ (in a water-based solution) as theelectrolyte.

In one example, nano-structure template 80 is formed by anodization ofthe second oxidizable material layer 60 in a 4% solution of oxalic acid(C₂H₂O₄), at a voltage of 30 Volts until substantially the entire Allayer is consumed. For a suitably thick Al layer, the resultingnano-structure template 80 will define nano-pores 82 that areapproximately 30 nanometers wide, and that will allow oxidation ofunderlying first oxidizable material layer 50. The nano-structuretemplate should have a template height (h_(T)) sufficient to allowcomplete growth of a nano-pillars 40 (including both stem portions 42and cap portions 44) within the nano-pores, as described below.

After the first anodization process, the nano-pore diameter (d_(p1)) maybe further tuned to a target nano-pore diameter by anisotropic etching,or other suitable process. Anisotropic etching may be performed usingdiluted phosphoric acid (5 vol. %). The time for etching may vary,depending, at least in part, upon the desirable average diameter for thefinal pores. The temperature for etching may also depend upon theprocess, the etching rate, and the etchant used.

In some examples, prior to performing the first anodization process, thefirst oxidizable material layer may be patterned to precisely definelocations of nano-pores 82 in the resulting nano-structure template 80.Patterning may be accomplished via any suitable technique. The patternedlayer (not shown) is then anodized, for example, by employing thepatterned layer as the anode of an electrolytic cell. A suitable amountof voltage and current is then applied to the electrolytic cell for anamount of time to completely anodize the patterned layer in accordancewith the first anodization process described above. This can result insubstantially uniformly spaced nano-structures where the variance inspacing between nano-structures differs by less than 1% (for nanometerscale dimensions).

Referring now to FIG. 4C, nano-pores 82 may be partially filled todefine nano-pillar stem portions 42. Nano-pillar stem portions may beformed via a second anodization process selected to partially anodizethe underlying first oxidizable material layer 50 (e.g., the Ta layer).Such second anodization process will grow an oxide from the firstoxidizable material, the oxide forming in the nano-pores 82 of thenano-structure template 80 from the bottom up. Where the firstoxidizable material layer 50 is formed of a metal such as tantalum (Ta),the resulting oxide may take the form of a dense oxide such as anodictantalum pentoxide (Ta₂O₅).

The second anodization process may be accomplished, for example, using aprocess similar to the first anodization process described above. Morespecifically, the first oxidizable material layer 50 is anodized byemploying the first oxidizable material layer as the anode of anelectrolytic cell to achieve a desired oxidation of the first oxidizablematerial.

For oxidation of tantalum, non-limiting examples of electrolyte mayinclude solutions containing citric acid (C₆H₈O₇), oxalic acid (C₂H₂O₄),boric acid (H₃BO₃), ammonium pentaborate ((NH₄)₂B₁₀O₁₆×8H₂O), and/orammonium tartrate (H₄NO₂CCH(OH)CH(OH)CO₂NH₄). It is to be understoodthat this type of anodization forms a dense oxide, where both theinterface between the remaining first oxidizable material and the formedoxide, and the interface between the formed oxide and the electrolyteare planarized.

During anodization of the first oxidizable material layer 50 (in thisexample, a tantalum layer), the formed oxide (in this example, tantalumpentoxide (Ta₂O₅)) grows through the individual nano-pores 82 defined innano-structure template 80 to form a nano-pillar stem portion 42 in eachnano-pore. The orientation of nano-pillar stem portions 42 is generallycontrolled by the orientation of the nano-pores 82. In the presentexample, the nano-pillar stem portions 42 are substantially orthogonalto the surface 22 of substrate 20.

The expansion coefficient of a material to be oxidized is defined as theratio of oxide volume to consumed material volume. The expansioncoefficient for oxidation of tantalum (Ta) is approximately 2.3.Accordingly, in the present example, due to the significant expansion oftantalum pentoxide (Ta₂O₅), and the fact that the resulting oxide(Ta₂O₅) is dense, the nano-pores 82 are filled from the bottom up. Itwill be understood that although the first oxidizable material istantalum (Ta) in the present example, other materials with an expansioncoefficient greater than 1 would similarly allow the oxidizable materialto squeeze into the nano-pores 82 of template 80.

As indicated, the grown oxide will partially fill nano-pores 82 ofnano-structure template 80 to define nano-pillar stem portions 42. Thegeometries of the nano-pillar stem portions 42 will substantiallyconform to the geometries of corresponding nano-pores 82, within whichthe nano-pillar stem portions are growing. Nano-pillar stem portions 42thus may take the form of substantially uniform cylindrical columns,substantially orthogonal to substrate surface 22, and substantiallyuniformly spaced across the substrate surface.

In the present example, each nano-pillar stem portion has asubstantially uniform stem thickness (indicated as stem diameter (d1))that corresponds to the nano-pore diameter (d_(p1)). Nano-pillar stemportions 42 are grown to a stem height (h1) that is less than templateheight (h_(T)) so as to allow subsequent growth of nano-pillar capportions 44. As shown, some residual first oxidizable material willremain beneath the grown oxide after the second anodization process(FIG. 4C). This residual first oxidizable material may subsequently beused to grow nano-pillar cap portions 44.

The geometry and/or dimensions of the nano-pillar stem portions 42 mayfurther be controlled by adjusting one or more parameters of theanodization process. For example, the stem height (h1) will depend onthe anodization voltage applied to the first oxidizable material layer50 during its anodization. In some examples, nano-pillar stem portionsare formed by anodizing the first oxidizable material at a first voltagecorresponding to a target nano-pillar stem portion height.

In one example, nano-pillar stem portions having a stem height (h1) of90 nanometers (at a stem diameter of approximately 30 nanometers) may beformed by anodization of Ta layer 50 in a 0.1% solution of citric acid(C₆H₈O₇), at a current density of 2 mA/cm² until voltage reaches 55V,and for 5 minutes more at 55V. It will be appreciated that stem height(h1) may be tuned to a target stem height by selecting a correspondinganodization voltage. For example, nano-pillar stem portions having astem height of 155 nanometers may be formed by anodization of Ta layer50 in a 0.1% solution of citric acid (C₆H₈O₇ at a current density of 2mA/cm² until voltage reaches 100V, and for 5 minutes more at 100V.

As indicated in FIG. 4D, once the nano-pillar stem portions are grown tothe target stem height (h1), the nano-pores 82 may be re-shaped todefine re-shaped nano-pores 82′ with separate stem-forming sections 82 aand cap-forming sections 82 b. In the depicted example, the nano-poresremain substantially unchanged in stem-forming sections 82 a, but arebroadened in cap-forming sections 82 b, thereby providing for subsequentformation of nano-pillar cap portions 44 that are wider than previouslyformed nano-pillar stem portions 42. As indicated, the re-shapednano-pores 82′ have a first width (indicated as original nano-porediameter (d_(pi))) in stem-forming sections 82 a, and a second,different width (indicated as modified nano-pore diameter (d_(p2)) incap-forming sections 82 b. The modified nano-pore diameter (d_(p2)) isgreater than the original nano-pore diameter (d_(p1)).

In some examples, nano-pillars 82 are re-shaped by broadening unfilledsections of the nano-pores 82 (the sections of the nano-pores above theformed stem portions 42). Such broadening may be achieved by selectiveetching of the nano-structure template 80. Selective etching may beaccomplished by employing an etchant solution configured to etch theexposed areas of porous oxide forming the nano-structure template 80(e.g., anodic porous alumina, Al₂O₃) at a rate that is substantiallyhigher than the etch rate for the oxide of the first oxidizable material(e.g., anodic tantalum pentoxide (Ta₂O₅)).

In one example, porous alumina nano-structure template 80 (withnano-pores that are approximately 30 nanometers wide) is etched in a 5%solution of phosphoric acid (H₃PO₄) at a temperature of 30° C. forapproximately 15 minutes to broaden the nano-pores to a modifiednano-pore diameter (d_(p2)) of approximately 60 nanometers. In anotherexample, the porous alumina nano-structure template 80 is etched in a 5%solution of phosphoric acid (H₃PO₄) at a temperature of 30° C. forapproximately 30 minutes to broaden the nano-pores to a modifiednano-pore diameter (d_(p2)) of approximately 80 nanometers. It thus willbe appreciated that the width of the broadened sections (cap-formingsections 82 b) may be tuned to a target width by selecting an etchduration corresponding to the target width. The target width may beselected to accommodate formation of nano-pillar cap portions suitableto serve as anchors for mechanically securing a cover layer to thesubstrate 20, as will be described further below.

Referring now to FIG. 4E, it will be seen that the cap-forming sections82 b of re-shaped nano-pores 82′ may be at least partially filled todefine nano-pillar cap portions 44 contiguous with nano-pillar stemportions 42. In some examples, the nano-pillar cap portions may beformed via a third anodization process selected to anodize the residualfirst oxidizable material (e.g., the remaining Ta layer) to continue theprocess of growing oxide into the (now) re-shaped nano-pores 82′.

As described generally above, the third anodization process will growoxide into the re-shaped nano-pores 82′ from the bottom up. Theresulting oxide thus will cause previously formed oxide to grow from thestem-forming sections 82 a into the cap-forming sections 82 b. The thirdanodization process may be substantially the same as the secondanodization process, but at an anodization voltage corresponding to atarget nano-pillar cap portion height.

More specifically, the first oxidizable material layer 50 is againanodized by employing the first oxidizable material layer as the anodeof an electrolytic cell, and applying a suitable amount of ananodization voltage and current to the first oxidizable material layerto achieve a desired oxidation. As described above, non-limitingexamples of electrolyte for oxidation of tantalum (Ta) include solutionscontaining citric acid (C₆H₈O₇), oxalic acid (C₂H₂O₄), boric acid(H₃BO₃), ammonium pentaborate ((NH₄)₂B₁₀O₁₆×8H₂O), and/or ammoniumtartrate (H₄NO₂CCH(OH)CH(OH)CO₂NH₄). The electrolyte may be present, forexample, in a water-based solution.

Again, anodization of the Ta layer will be understood to form a denseoxide (in this example, tantalum pentoxide (Ta₂O₅)), where both theinterface between the remaining first oxidizable material and the formedoxide, and the interface between the dense oxide and the electrolyte areplanarized.

Because orientation of nano-pillars is generally controlled by theorientation of the re-shaped nano-pores 82′, where the re-shapednano-pores are orthogonal to surface 22 of substrate 20, the fully grownnano-pillar stem portions 42 and nano-pillar cap portions 44 aresubstantially orthogonal to surface 22 of substrate 20 (shown in FIG.4F). It also will be appreciated that the cap portions 44 take the formof substantially uniform cylindrical columns, conforming to the shape ofcap-forming sections 82 b.

In the present example, each nano-pillar cap portion 44 has asubstantially uniform cap thickness (indicated as cap diameter (d2))that corresponds to the nano-pore diameter (d_(p2)). Nano-pillar capportions 42 are grown to a cap height (h2), providing nano-pillars ofoverall height (H), where H=h1+h2. As shown, some residual firstoxidizable material will remain beneath the grown oxide after the secondanodization process (FIG. 4C).

Nano-pillar cap portions having a cap height (h2) of approximately 100nanometers (at a cap diameter (d2) of approximately 60 nanometers) maybe formed by anodization of Ta layer 50 in a 0.1% solution of citricacid (C₆H₈O₇), at a current density of 2 mA/cm² until voltage reaches200V, and for 5 minutes more at 200V. Cap height (h2) may be tuned to adifferent target cap height by selecting a different final anodizationvoltage.

In FIG. 4F, the nano-structure template 80 is removed to expose thefully formed capped nano-structures 40. The nano-structure template 80may be removed using a second selective etching process that will removethe nano-structure template 80 without deleteriously affecting thenano-pillars 40, or other features of article 10. In one example, theselective etching may be performed using a selective etchant containingH₃PO₄ (92 g), CrO₃ (32 g) and H₂O (200 g), at approximately 95° C. Ithas been found that the example tantalum pentoxide (Ta₂O₅) nano-pillars40 can withstand this particular etching process for more than one hour,while the example anodic porous alumina (Al₂O₃) nano-structure template80 is etched away at a rate of about 1 micron per minute. Otherselective etchants are also contemplated, dependent on the particularcharacteristics of the nano-structures.

As indicated in FIG. 4G, a cover layer 35 may be applied to thenano-structured surface. The cover layer typically is applied as aflowable material of suitable viscosity to allow flow of the materialbetween and around nano-structures 40. In some embodiments, the coverlayer may be formed of SU-8, a curable epoxy that has been shown toprovide excellent adhesion with tantalum pentoxide (Ta₂O₅). SU-8 also issuitably flowable to allow flow of material between and aroundnano-structures 40 prior to solidification.

Once applied, the flowable material (e.g., SU-8) is solidified,establishing a cover layer 35 that may chemically bond to the surface ofsubstrate 20. The increased surface area of the nano-structured surfaceprovides for enhanced chemical bonding between cover layer 35 andsubstrate 20. As indicated, the nanostructures also provide a mechanicalanchor between the cover layer and the substrate, an interface region 36of the cover layer being interwoven with the nano-structured surface ofthe substrate. The projecting nano-structures (both stem portions andcap portions) will oppose shear forces (indicated by arrow F1) betweenthe cover layer and the substrate. The cap portions will oppose forcesnormal to the substrate (indicated by arrow F2).

Nano-structured surfaces such as those described herein provideexcellent adhesive properties, particularly in wet environments, whereinterfaces are more inclined to fail. For example, shear strength of aninterface between an adhesive cover layer (EMS 357-243-2 manufactured byEngineered Materials Systems, Inc.) and an LCP/PPS plastic substratewith tantalum pentoxide (Ta₂O₅) nano-structures will not be appreciablyaffected by ink soak (at 70° C.) for 2 weeks, or even 4 weeks. For asurface coated with tantalum, but without tantalum pentoxidenano-structures, shear strength of the SU-8/tantalum interface maydecrease by as much as 70% or more after ink soak (at 70 degrees) for 2weeks.

FIG. 5 shows a high-level flowchart 500 of a method of adhering a coverlayer to a substrate, as described herein. The method generallyincludes: 1) forming an array of nano-structures on a substrate; 2)applying a flowable material to the substrate, the flowable materialsubstantially enveloping the nano-structures on the substrate; and 3)solidifying the flowable material to form a cover layer on thesubstrate, the cover layer being anchored to the substrate via thenano-structures.

More particularly, at 510, a template is formed on the substrate, thetemplate defining nano-pores having a first width. The template may beformed by anodizing a layer of oxidizable material on the substrate. At520, the nano-pores are partially filled to define nano-pillar stemportions having a first thickness corresponding to the first witdth ofthe nano-pores. The nano-pillar stem portions may be formed by anodizinga layer of another oxidizable material disposed on the substrate,beneath the template, to grow an oxide into the nano-pores of thetemplate.

At 530, the nano-pores are re-shaped to define re-shaped nano-poresections having a second width greater than the first width. Re-shapingthe nano-pores may include selective etching of nano-pores sections thatdo not include nano-pillar stem portions. At 540, the re-shapednano-pores are at least partially filled to define nano-pillar capportions on the stem portions, the cap portions having a secondthickness corresponding to the second width of the re-shaped nano-poresections. The nano-pillar cap portions may be formed by furtheranodizing the layer of another oxidizable material disposed beneath thetemplate, to grow oxide into the re-shaped nano-pore sections. At 550,the template is removed. Removal of the template will reveal fullyformed integral nano-pillars including stem portions and cap portions.

At 560, a flowable material is applied to the substrate, the flowablematerial flowing between the nanostructures, and substantiallyenveloping the nano-structures on the substrate. At 570, the flowablematerial is solidified (e.g., by curing) to form a cover layer on thesubstrate, the cover layer being mechanically anchored to the substratevia the nano-structures. In some embodiments the cover layer also may bechemically anchored to the substrate via chemical bond between theflowable material and the nano-structures upon solidifying the flowablematerial.

Anodizing the first oxidizable material may include anodizing the firstoxidizable material at a first voltage corresponding to a targetnano-pillar stem portion height. Similarly, further anodizing the firstoxidizable material may include further anodizing the first oxidizablematerial at a second voltage corresponding to a target nano-pillar capportion height. Broadening unfilled sections of the nano-pores mayinclude etching of the substrate in an etchant solution configured toetch the porous oxide at a substantially higher etch rate than the oxideof the first oxidizable material.

Although the present invention has been described with reference tocertain representative examples, various modifications may be made tothese representative examples without departing from the scope of theappended claims.

What is claimed is:
 1. An article, comprising: a substrate having asurface; a nano-structure array formed on the substrate, thenano-structure array including a plurality of nano-structures extendingfrom the surface of the substrate; and a cover layer formed on andaround the nano-structures to anchor the cover layer to the substrate.2. The article of claim 1, wherein the nano-structures include stemportions extending from the surface of the substrate and having a firstthickness, and include cap portions extending from the stem portions andhaving a second thickness greater than the first thickness tomechanically anchor the cover layer to the substrate.
 3. The article ofclaim 1, wherein the substrate includes a first surface region with afirst set of nano-structures extending in a first direction and a secondsurface region with a second set of nano-structures extending in asecond direction intersecting with the first direction.
 4. The articleof claim 3, wherein the first set of nano-structures are substantiallyorthogonal to the first surface region and the second set ofnano-structures are substantially orthogonal to the second surfaceregion.
 5. The article of claim 1, wherein the cover layer is formed ofa flowable material that substantially envelops the nano-structuresbefore being solidified to secure the cover layer to the substrate.
 6. Amethod of adhering a cover layer to a substrate, the method comprising:forming an array of nano-structures on a substrate; applying a flowablematerial to the substrate, the flowable material substantiallyenveloping the nano-structures on the substrate; and solidifying theflowable material to form a cover layer on the substrate, the coverlayer being anchored to the substrate via the nano-structures.
 7. Themethod of claim 6, wherein forming an array of nano-structures includes:forming a template on the substrate, the template defining nano-poreshaving a first width; partially filling the nano-pores to define stemportions of a first thickness corresponding to the first width;re-shaping the nano-pores to define re-shaped nano-pore sections havinga second width greater than the first width; at least partially fillingthe re-shaped nano-pore sections to define cap portions of a secondthickness corresponding to the second width; and removing the template.8. The method of claim 7, wherein partially filling the nano-poresincludes: forming a layer of a first oxidizable material; and anodizingthe layer of first oxidizable material to grow oxide from the firstoxidizable material into the nano-pores.
 9. The method of claim 8,wherein at least partially filling the re-shaped nano-pore sectionsincludes further anodizing the first oxidizable material to grow oxideinto the re-shaped nano-pore sections.
 10. The method of claim 9,wherein forming a template includes: forming a layer of a secondoxidizable material; and anodizing the layer of second oxidizablematerial to define the nano-pores.
 11. The method of claim 6, whereinforming an array of nano-structures on a substrate includes forming afirst set of orthogonal nano-structures on a first surface region andforming a second set of orthogonal nano-structures on a second surfaceregion, the first region intersecting the second region such that thecover layer is locked in place upon solidifying the flowable material toform the cover layer.
 12. A method of adhering a cover layer to asubstrate, the method comprising: depositing a first oxidizable materialonto the substrate; depositing a second oxidizable material onto thefirst oxidizable material; anodizing the second oxidizable material toform a porous oxide having nano-pores in the porous oxide; anodizing thefirst oxidizable material so as grow an oxide of the first oxidizablematerial into the nano-pores; removing the porous oxide, therebyyielding an array of spaced nano-structures extending from thesubstrate; applying a flowable material to the substrate, the flowablematerial flowing between the spaced nano-structures; and solidifying theflowable material to form a cover layer on the substrate, the coverlayer being anchored to the substrate via the nano-structures.
 13. Themethod of claim 12, wherein anodizing the first oxidizable materialincludes growing a first set of nano-structures on a first surfaceregion of the substrate and growing a second set of nano-structures on asecond surface region of the substrate, the first region intersectingthe second region such that the cover layer is locked in place uponsolidifying the flowable material to form the cover layer.
 14. Themethod of claim 12, wherein anodizing the first oxidizable materialincludes growing an oxide into the nano-pores to define stem portionshaving a first width, broadening cap-forming sections of the nano-pores,and growing oxide into the nano-pores to define cap portions having asecond width greater than the first width, thereby definingnano-structures having cap portions that anchor the cover layer to thesubstrate upon solidifying the flowable material to form the coverlayer.
 15. The method of claim 12, wherein solidifying the flowablematerial includes chemically bonding the cover layer to thenano-structures.